Add salad dressing ingredients to a pint-sized glass mason jar. Muddle the blueberries, onion and thyme until most of the berries have burst, then seal the jar. Vigorously shake the jar of dressing until fully combine. Set aside.
Wash and dry arugula and scatter onto a large platter.
Chop pistachios and set aside.
Slice the block of halloumi cheese into 1/4" thick slices. Heat a grill to 400°F and grill the cheese on each side for 60-75 seconds or until nice grill marks form. Remove the cheese from the grill with a spatula.
Layer the grilled cheese over the platter of arugula. Remove the springs of thyme from the dressing, give the dressing one more shake, then spoon the dressing and marinated onions and blueberries over the cheese and greens. Top the salad with the chopped pistachios and a handful of fresh blueberries. Serve immediately.
This salad serves 4 as a main dish or 6 as a side dish.